Toggle output/argument expansion.
A click on the header/args region (the truncated command or code line
and its hint) toggles the collapsible args/code block directly, so an
execute command or js_eval program can be expanded even when the
output below it is also expandable. Otherwise prefer toggling output,
falling through to the args/code block only when the output can't
expand — js_eval commonly has a short, unexpandable result sitting
below a multi-line, collapsible code block, and the old
"output wins whenever it exists" rule left that code block stuck.
